1. Field of the Invention
The present invention relates to solder alloy compositions, more particularly to a low melting point, lead-free solder alloy composition provided with improved properties, primarily in mechanical strength.
2. Description of the Related Art
When electronic parts, etc., are electrically connected to and are fixed on circuit boards and the like, a Sn--Pb eutectic solder is most widely used at present. The Sn--Pb solder has a eutectic alloy composition of 63 percent by weight of Sn and 37 percent by weight of Pb, and has a eutectic temperature of 183.degree. C. The Sn--Pb eutectic solder alloy composition exhibits superior performance in many respects, such as temperature characteristics, soldering characteristics, and reliability, and forms the basis of mounting technology in the electronics industry.
Recently, wastes containing Pb have become a problem from the point of view of environmental protection and the enactment of lead-control regulations is anticipated in the United States, Europe, and Japan. The Sn--Pb eutectic solder will also be regulated.
Accordingly, a lead-free solder is desired, which has equivalent performance to that of the Sn--Pb eutectic solder in workability, such as soldering characteristics, temperature characteristics, and reliability.
Alloy compositions proposed up to the present as candidates for a lead-free solder are mainly composed of Sn, such as low melting point alloys including a Sn--Bi alloy (57 percent by weight of Bi and a eutectic temperature of 139.degree. C.) and a Sn--In alloy (52 percent by weight of In and a eutectic temperature of 117.degree. C.). In addition, there is a Sn--Ag alloy with a relatively high melting point (3.5 percent by weight of Ag and a eutectic temperature of 221.degree. C.).
When compared with the Sn--Pb eutectic solder, however, all compositions described above have problems, such as decrease of wettability, decrease of fatigue strength, generation of dross (an oxide film on surfaces of molten solder), and higher cost. Among the compositions described above, the Sn--Ag alloy, even though it has a drawback of having a relatively higher melting point, is regarded as the most likely candidate by virtue of superior properties of oxidation resistance and soldering characteristics. A phase diagram of the Sn--Ag alloy is shown in the FIGURE.
A Sn--Ag alloy having Bi and Cu, 90Sn-7.5Bi-2Ag-0.5Cu (each numeral represents percent by weight), is named Alloy-H and the melting point thereof decreases to approximately 210.degree. C. While having good thermal cycle characteristics, Alloy-H is hard and brittle due to a large amount of Bi contained, so that it is difficult to make a solder wire. A problem of lift-off is also noted.
Consequently, the inventors of the present invention proposed a lead-free Sn--Ag alloy solder disclosed in Japanese Patent Application No. 9-348212, in which the Bi content was reduced to approximately half, that is, 4 percent by weight, and a small amount of Ge was added to compensate for the reduced workability and reliability. A typical alloy composition is a five-component alloy of 93.4Sn-2Ag-4Bi-0.5Cu-0.1Ge ( each numeral represents percent by weight).
The five-component lead-free alloy solder shows superior properties; however, degradation of properties is observed when used in a flow process and an elongation thereof is relatively small. Moreover, application of the alloy solder was limited by the extremely high cost of Ge.